diff --git a/AstroAtmosphere/refractivityModels.py b/AstroAtmosphere/refractivityModels.py
index 3787ea0..ece68f8 100644
--- a/AstroAtmosphere/refractivityModels.py
+++ b/AstroAtmosphere/refractivityModels.py
@@ -244,10 +244,10 @@ def BonschPotulski(l, T=288.15, p=101325, RH=0, CO2=400, f=None):
 	return n
 
 
-def BirchDowns(l):
+def BirchDowns(l, T=288.15, p=101325, RH=0, f=None):
 	'''
 		From Birch & Downs, 1993 and 1994 (correction)
-		Equations 1 and 2
+		Equations 1, 2 and 3
 
 		Only at standard conditions
 
@@ -255,15 +255,32 @@ def BirchDowns(l):
 		----------
 		l 	: 	float
 			wavelength in micron
+		T 	: 	float
+			Temperature in Kelvin
+		p 	: 	float
+			Atmospheric pressure in Pa
+		RH 	: 	float
+			Relative humidity (0<=H<=1)
+		f 	: 	float
+			Water vapor pressure in Pascal
 
 		Returns
 		-------
 		n 	: 	float
 			Refractive index of atmospheric air at the given conditions
 	'''
+	Tc = K2C(T)
 	s = 1 / l
-	R = 8342.54 + 2406147 / (130 - s**2) + 15998 / (38.9-s**2)
-	n = R/1e8 + 1
+	Rs = 8342.54 + 2406147 / (130 - s**2) + 15998 / (38.9-s**2)
+	n = Rs/1e8 + 1
+
+	R_tp = (p * Rs/1e8) / 96095.43 * (1 + 1e-8 * (0.601 - 0.00972 * Tc) * p) / (1 + 0.0036610 * Tc) 
+	n_tp = R_tp + 1
+	
+	if f is None:
+		f = RH * svp(T)
+
+	n = n_tp - f * (3.7345 - 0.0401 * s**2) * 1e-10
 	return n